How to create segment a large model of landscape into smaller sections that can be higher resolution?
To answer your question. Slice down your original hight map into smaller parts. So you can assemble multiple so called map tiles into one high resolution map.

For large detailed enviroment usualy tiling maps are used in game engines. The heightmaps are loaded directly in a shader - in modern games with adaptive generation of geometry depending on the distance you can actualy see (Tesselation). 
If you generate a high resolution mesh for each map tile you most likeley will run into memory issues.